CHAPTER 2 LITERATURE REVIEW

Transcription

1 25 CHAPTER 2 LITERATURE REVIEW The performance of a heat pipe is critical, and is often judged in part by the amount of heat a unit length of the heat pipe can transport under a uniform heat load. Many investigations have been performed concerning heat pipe operating limits, heat pipe applications and design modifications to improve the heat pipe performance. In the present work, a detailed review has been made on the various research works carried out experimentally and theoretically on the operating limits, startup considerations and various design parametric considerations. 2.1 STARTUP CONSIDERATION Jang et al (1991) mathematically developed a model, to predict the start up behavior of the heat pipe from the frozen state condition. A parametric study is performed to examine the effects of the boundary specification at the surface of the outer wall on the successful start up from the frozen state. Cao and Faghri (1991) numerically analysed the transient performance of a leading edge heat pipe with high heat fluxes. The leading edge heat pipe reached the steady state very quickly, and the vapor pressure drop was a dominant factor for the capillary limit consideration. Even though the vapor temperature was relatively uniform along the heat pipe length, large temperature gradients existed at the outer wall surface of the heat pipe. Cao and Faghri (1993) also numerically studied the early start up period of a high temperature heat pipe from the frozen state, by applying the rarefied vapor self diffusion model. Issacci et al (1991) developed a vapor flow model to

2 26 analyse the start up vapor dynamics in heat pipes for high and low input heat fluxes. For supplying high input heat flux in the evaporator section multiple wave reflections are created in the evaporator region. These wave reflections cause a significant increase in the local pressure, and a large pressure drop along the heat pipe. Furthermore, wave reflections cause flow reversal in the evaporator region, and flow circulations in the adiabatic region. Wang and Vafai (2000) developed an analytical model, and predicted the transient performance of a flat plate heat pipe for startup and shutdown operations. Faghri et al (1991) experimentally investigated the start up behavior of the heat pipe from the frozen state for various heat loads and input locations, with both low and high heat rejection rates at the condenser. The heat pipe is made of stainless steel, and the working fluid used was sodium. They concluded that the start up behavior of a liquid metal heat pipe from the frozen state was greatly dependent upon the heat rejection rate at the condenser. 2.2 OPERATING LIMITS Kemme (1969) investigated the maximum heat transfer limitation in heat pipes for sodium, potassium and cesium working fluids. The heat pipe consisted of an inner porous tube, an annulus for liquid return, and an outer container tube. Thin, rigid tubes with very small pores were obtained by compressing several layers of fine mesh screen. These tubes allowed large capillary forces to develop. Chun (1972) determined the dry out limit of the screen wick vertically pumping against gravity. The working fluid used was acetone. It also measured the thermal resistance of several sub layers (i.e 2, 3, 5 and 7) with various heat inputs in the evaporator section. Pruzan et al (1990) investigated the steady state heat flux in an everted heat pipe with

3 27 various working fluid compositions. The working fluids are pure water, an ethanol water mixture and pure ethanol. Joseph Schmalhofer and Amir Faghri (1991) experimentally analysed the transient and steady state performance of a copper water heat pipe, and the capillary limit for both modes of heating, such as Block heated and Circumferential heated under a step heat input of 50 Watts and 150 Watts in the evaporator section. They concluded that the transient and steady state performance of the heat pipe is the same for both modes of heating. The capillary limit for the block heated mode is higher than that of the circumferential heating mode, because of the shorter adiabatic length. Faghri and Buchko (1991) experimentally and numerically analysed the capillary limit of a copper water heat pipe with various positions of heat flux in the evaporator section. They concluded that the capillary limit is lower when the position of heat flux is farthest from the condenser section. They also describe that increasing the heat flux in the evaporator section decreases the capillary limit. Moreover, the temperature distribution of the surface and vapor increases with the increase in the power input in the evaporator section. Ivan Catton and Stroes (2002) analytically predicted the wetted length capillary limit in inclined triangular grooves for a variety of operating conditions. The concept of the accommodation theory is introduced to account for the change in the radius of curvature of the liquid vapor interface between the liquid reservoir and the groove. Muraoka et al (2001) mathematically analysed the operational characteristics and limits of a loop heat pipe as a function of the heat load, at the evaporator, and the heat sink temperature, at the condenser. Also, the mechanism of failure at different operational modes was investigated. A detailed literature review on heat pipes was carried out by Stephane Launay et al (2007). This paper discusses the operating limits of a loop heat pipe with various choices of the working fluid, the fill charge ratio, the porous wick geometry and thermal properties, the sink and ambient

4 28 temperature levels, the design of the evaporator and compensation chamber, the elevation and tilt, the presence of non condensable gases, and the pressure drop of the fluid along the loop. Shinzo Shibayama and Shinichi Morooka (1979) experimentally and theoretically studied the capillary limit, such as the maximum heat transfer limit in a heat pipe with respect to wick characteristics, friction losses and capillary properties. Pruzan et al (1990) analytically predicted the steady state heat flux limits in a sintered wick heat pipe, with various geometrical parameters in the wick structure such as the wick thickness, effective capillary radius of curvature, porosity and heated wire diameter. The analytical results are compared with the experimental results. They concluded that the dry out heat flux increases with the increase in all geometrical parameters in the wick structure, except in the tilt angle. The dry out heat flux decreases with an increase in the tilt angle. Kim and Peterson (1994) investigated the entrainment phenomenon in a capillary driven heat pipe, experimentally and analytically. Moreover, a computer model was developed to investigate the capillary limitation, entrainment limit and boiling limitation at different mesh numbers in the adiabatic region and at different vapor temperatures. 2.3 PERFORMANCE INVESTIGATIONS Theoretical Work Bankston and Smith (1973) described the flow of vapor in a cylindrical heat pipe with various evaporator Reynolds number and condenser Reynolds number. The results obtained were used to solve the complete axisymmetric Navier Stokes equation for the steady, laminar vapor flow in circular heat pipes with various lengths of evaporator and condenser. Rohani and Tien (1973) analysed the performance of a gas loaded heat pipe with different vapor gas mixtures such as, water air, and sodium

5 29 argon. In this investigation, the heat conduction through the pipe wall, and the liquid wick is negligible, compared to the heat transfer due to the latent heat of the vapor diffusing into the non condensible gas region. Faghri et al (1989) numerically investigated the Nusselt number, interface temperature, and interfacial heat flux, by varying the thermal conductivity ratio and tube wall thickness. Amir Faghri (1989) numerically analysed the pressure drop in the evaporator and condenser section of the concentric annular heat pipe, with a different flow models. Faghri and Chen (1989) numerically analysed the conjugate heat transfer, vapor compressibility and viscous dissipation in heat pipes, with different thermal conductivity ratios, and evaporators, and condenser radial Reynolds number model with sodium and water as the working fluids. The results showed that if the thermal conductivity ratio is increased, the interfacial heat flux varies due to the axial conduction. The difference between the compressible and incompressible models in compressibility effects, such as outer wall temperature distribution, liquid and vapor pressure drop and mach number is high, at a higher evaporator radial Reynolds number. The maximum pressure recovery and flow reversal occur at a higher condenser radial Reynolds number. Chen and Faghri (1990) studied both single and multiple heat sources albeit in a two dimensional axisymmetric cylindrical heat pipe. A coupled analysis of the wall, wick and vapor regions was conducted. Both sodium and water were considered as the working fluids. The solutions were compared against the experimental results for the vapor and wall temperature at high and low operating temperatures for the operating conditions considered. The compressibility effects were found to be very important. Hall and Doster (1990) modeled the heat pipe to evaluate the sensitivity of the heat pipe with various accommodation coefficients of the

6 30 fluid in the evaporator and condenser sections. The working fluid was lithium and air was the noncondensable gas. The results concluded that as the accommodation coefficient increases, the heat transfer rate also increases. Faghri et al (1991) numerically analysed the transient and steady state performance of heat pipes with multiple heat sources and sinks. The results concluded that the steady state of the heat pipe significantly changes with a change in the emissivity of the heat pipe wall and subsequently increases the power input in the evaporator section. Schmalhofer and Faghri (1993) numerically developed a block heated and circumferentially heated heat pipe. The operating characteristics of a block heated and circumferentially heated heat pipe are studied numerically. The results concluded that the operating characteristics are similar in both modes of heating. In the circumferential heating mode the maximum vapor velocity occurs near the center of the heat pipe. In the case of the block heating mode, the maximum vapor velocity occurs near the liquid wick region. Salinas and Marto (1991) numerically studied the heat transfer rate of a coaxial rotating heat pipe with various effects of parameters such as the number of fins, rotational speed, magnitude of the out side heat transfer coefficient and different working fluids of water and Freon 113. The results concluded that the increase in the above parameters increased the heat transfer rate. The heat transfer rate is higher for water, and so is used as a working fluid than Freon-113. Tournier and El Genk (1993) theoretically analysed the transient behavior of a heat pipe with the various radii of curvature at the liquid vapor interface in the evaporator and condenser sections under different conditions of heat up and cool down periods of the heat pipe. Longtin et al (1994)

7 31 numerically analysed the pressure, velocity, and interfacial curvature as a function of the axial distance along the micro heat pipe during the steady state condition. Kim and Peterson (1994) theoretically and experimentally predicted the critical air / vapor velocities on different entrainment phenomena in capillary driven heat pipes. The results concluded that the critical air/ vapor velocities are higher for wave and intermediate entrainment phenomena, and lower for shear induced entrainment phenomena. Zhu and Vafai (1995) developed a pseudo three dimensional analytical model and investigated the effect of liquid vapor coupling and boundary and inertial effect on a asymmetrical disk shaped heat pipe. The incompressible fluid of vapor and the liquid flow in a heat pipe during the steady state condition the heat transfer performance are analysed. Zhu and Vafai (1997) numerically and analytically analysed the characteristics of the vapor flow in an asymmetrical disk shaped heat pipe. Peterson and Ma (1996) mathematically predicted the minimum meniscus radius and the maximum heat transport in a micro heat pipe, based on the physical characteristics and geometry of the capillary grooves. Tournier and El Genk (1996) numerically analysed the transient behavior of a liquid metal heat pipe, with different vapor flow models, such as the free molecular vapor flow, and the transition and continuum vapor flow models. The results concluded that the flow is free molecular and vapor flow the heat transport capacity is lower. In the case of the flow is continuum flow the heat transport capacity is higher. Zuo and Faghri (1997) theoretically analysed the transient behavior of a heat pipe. Ha and Peterson (1998) analytically predicted the maximum heat transport capacity in a micro heat pipe. Tan et al (2000) analytically evaluated the liquid pressure and velocity distribution of a flat plate heat pipe with multiple heat sources. They also discussed the heat pipe performance with various positions of one, two, and four point heat sources on a flat plate heat pipe.

8 32 Song et al (2003) theoretically investigated the performance of a high speed rotating heat pipe with various working fluid loadings, rotational speeds, and heat pipe geometries. Moreover, the flow and heat transfer in the condenser is modeled by using the conventional modified Nusselt film condensation approach. Williams and Harris (2005) theoretically analysed the heat transfer limit of different wick structures, such as the two control metal felt wick and the two step graded metal felt wick. It also analyses the failure that may be caused from the vapor formation within step graded wicks. Vadakkan et al (2004) numerically analyzed the transient and steady state performance of flat heat pipes subjected to the input heat flux, and the spacing between the discrete heat sources is studied as a parameter. Balram Suman and Nazish Hoda (2005) theoretically studied the performance of a micro heat pipe with various contact angles for the substrate coolant liquid system, surface tension and viscosity of the coolant liquid, inclination, groove angle, length of the adiabatic section, and radius of the ungrooved substrate. Brian Holley and Amir Faghri (2005) theoretically investigated the enhancing heat transfer in a pulsating heat pipe, with various channel diameter profile gravities, fill ratios, and heating and cooling schemes. Mwaba et al (2006) numerically investigated the performance of a heat pipe with different wick structures such as coarse pore sizes, fine pore sizes and a composite comprised of coarse and fine pore sizes. Yongping Chen et al (2009) developed a theoretical model of a heat pipe with axial shaped micro grooves. The heat transfer performance was

9 33 analysed with different liquid vapor interfacial shear stress, variation of meniscus radius, contact angle, the vapor core and wick structure, heat load of evaporator, the wick size, and working temperatue. Min et al (2009) developed a multi artery heat pipe spreader, to optimize the artery geometry, number, and distribution, for both liquid and air cooled, finned condenser and show that the overall thermal resistance is substantially lower than the uniform wick vapor chamber Experimental work Daniels and Al Jumaily (1974) experimentally and theoretically investigated the performance of a rotating heat pipe with various rotational speeds, temperature differences across the condensate films, fluid properties and heat pipe geometry. Sun and Tien (1974) experimentally and theoretically evaluated the overall thermal performance of a single component and gas loaded heat pipe with two different working fluids, such as water and acetone, as well as two corresponding sink environments such as boiling water and boiling alcohol, and different heat inputs. Abhat and Seban (1974) investigated the performance of the heat pipe with different working fluids such as water, ethanol and acetone. The heat pipe consists of a wrapped screen or felt metal used as the wick material; this heat pipe is positioned vertically. Faghri and Thomas (1989) experimentally analysed the performance of a concentric annular heat pipe at the inner and outer wall of the evaporator section. The results were compared with those of a conventional heat pipe. It was found that the concentric annular heat pipe has a higher heat transport capacity at a higher heat load and zero tilt angle. This was attributed to the

10 34 increase in the cross sectional area of the wick in the concentric annular heat pipe as compared to the conventional heat pipe. Mohamed et al (1993) experimentally investigated the time constants of the vapor temperature, and the effective power throughput, for both heat up and cool down transients were determined as functions of the electric power input in the evaporator section, and the cooling water mass flow rate in the condenser section. Zhao and Avedisian (1997) experimentally studied the heat transfer rate of the copper plate fins supported by a copper heat pipe and copper solid rod. The primary variable is the height of the fin stack, while the fin pitch, air flow rate, and fin shape are fixed. Ma and Peterson (1996) experimentally investigated the maximum heat transport and unit effective area heat transport in a micro heat pipe with varying groove widths but identical apex angles, using methanol as the working fluid. Jie Wei et al (1997) experimentally and analytically evaluated the temperature distribution and thermal efficiencies, the effects of the pipe dimension and surface convection, and the heat dissipating capability of the planer heat pipe. Thomas et al (1998) experimentally examined the steady state performance of a helically grooved copper ethanol heat pipe as a function of the heat input and transverse body force field strength. Hopkins et al (1999) experimentally investigated the maximum heat transfer capabilities of a flat miniature heat pipe with various grooves, such as trapezoidal and axial rectangular micro capillary grooves. Wang and Vafai (2000) experimentally investigated the thermal performance of a flat plate heat pipe. They also

11 35 described the transient characteristics of the flat plate heat pipe, and the correlations for the maximum temperature rise and maximum temperature difference in terms of the input heat flux. Lanchao Lin et al (2002) experimentally investigated the thermal performance of a miniature heat pipe with different capillary structures of the heat pipe, such as partially and fully opened grooves, and different fill amounts of the working fluid, and different modes of heat source in the evaporator section. Sung Jin Kim et al (2003) experimentally and analytically investigated the maximum heat transport rate and the overall thermal resistance under steady state conditions with various effects of the liquid vapor interfacial shear stress, the contact angle, and the amount of the initial liquid charge have been considered in the proposed model. Xu et al (2005) experimentally analysed the bulk circulation flow and the bubble displacements, and the velocities of the methanol and water pulsating heat pipe with different power inputs in the evaporator section. Vikas Kumar et al (2007) experimentally and analytically evaluated the thermal performance of the heat pipe with different tilt angles and heating fluid inlet temperature at the evaporator section and analytically predicted the heat transfer coefficient at the evaporator and condenser section. Asias et al (2007) experimentally studied the influence of accelerations in a heat pipe. Sugumar and Tio (2009) experimentally investigated the effect of the thermo physical properties of the working fluid on the performance of a micro heat pipe with a triangular cross section. The different working fluids are water, hepthane, ammonia, methanol and ethanol for operating temperatures ranging from 20 C to 100 C.

12 STUDIES ON DIFFERENT MODES OF CONDENSER COOLING Cao and Faghri (1990) mathematically investigated the transient response of a heat pipe with various cooling conditions on the condenser section, such as convective and radiative cooling. Their results concluded that with convective cooling on the condenser section, the heat pipe attains a steady state in a shorter duration. In the case of radiative cooling the heat pipe attains the steady state after a longer duration. Harley and Faghri (1994) numerically analysed the transient and steady state behavior of non-condensable and condensable gas loaded heat pipes. They also described the effect of emissivity on the condenser section. The results concluded that the temperature, pressure and velocity distributions are largely decreased from the evaporator section to condenser section in the case of the heat pipe that consists of condensable gas. Increasing the emissivity on the condenser section increases the heat transfer coefficient on the condenser surface. Zhu and Vafai (1999) analytically predicted the vapor and liquid velocity and pressure distribution, the steady state vapor and wall temperature for a given input heat load in the evaporator region. They also described a convective boundary condition in the condenser section and the effects of the liquid vapor interfacial hydrodynamic coupling and non Darcian transport through the porous wick. 2.5 OBJECTIVE OF THE PRESENT WORK The performance of a heat pipe depends on several factors, such as the radial and axial thermal resistances at the evaporator, condenser and inside the heat pipes. The surface resistance on the condenser section is one of the important parameters which also decides the operating temperature of the heat

13 37 pipe. There are several applications in which there will be constraints in the operational temperature of the heat pipe. Under such circumstances, the increase in the heat transfer through an increase in the surface area or in the surface heat transfer coefficient in the condenser section, is essential to maintain the operational temperature within the constraint. The objective of the present study is the experimental investigation of the transient operation of a water heat pipe under different modes of condenser cooling in order to determine the surface resistance and to determine the effective thermal conductivity of the heat pipe, which is being used as an important parameter in commercial software, to analyze the performance of many electrical and electronic cooling systems using heat pipes. In the present work, three heat pipes of the same dimensions of 1 m length and 0.031m outer diameter were constructed with some modifications in the condenser section, in order to provide three different modes of cooling, viz, air cooling, water cooling and cooling with extended surfaces in the condenser section. Experiments are conducted to determine the surface and vapor temperature distribution at the steady and transient conditions for all the above said three modes of cooling in the condenser section. The results obtained from the above investigation are compared and presented in this work. In addition, the effective thermal conductivity of the heat pipe is also determined and reported. A CFD analysis was also carried out, and the results obtained under the steady state conditions are compared with the results obtained from the experiments and reported.